Conventionally, an overload safety device for preventing an overload from being applied during hoisting work has been mounted on a crane. Japanese Unexamined Patent Publication No. 2002-211884 discloses an example of the overload safety device.
The overload safety device disclosed in Japanese Unexamined Patent Publication No. 2002-211884 includes storing means in which a plurality of kinds of work performance corresponding to a plurality of states of a crane during hoisting work are stored. The storing means stores work performance of each of states during respective kinds of work such as jib work, boom work, and boom with jib work. Jib work is work for, in a state in which a jib is attached to a boom so as to extend from a boom distal end, hoisting a hoisted load from the distal end of the jib. The boom work is work for, in a state in which the jib is stored so as to extend along the boom, hoisting the hoisted load from the distal end of the boom. The boom with jib work is work for, in a state in which the jib is attached to the boom so as to extend from the boom distal end, hoisting the hoisted load from the distal end of the boom.
In the overload safety device, a state during the hoisting work of the crane is derived on the basis of a kind of work manually set using work setting means and a detection result by jib-storage detecting means for detecting whether the jib is in a stored state. Work performance corresponding to the derived state is selected from the plurality of kinds of work performance stored in the storing means. A limit value corresponding the length of the boom and detection values of a derricking angle and a slewing angle of the boom is calculated on the basis of the selected work performance. When a detected value of a load acting on the boom reaches the limit value, the overload safety device regulates operations of driving sections of the crane to thereby prevent an overload from being applied.
As explained above, in the overload safety device disclosed in Japanese Unexamined Patent Publication No. 2002-211884, a limit value is not uniformly set and a limit value corresponding to each of states during the hoisting work of the crane is set. Therefore, the crane can exhibit as high hoisting abilities as possible in the states during the hoisting work.
Incidentally, some crane is mounted with, as a winch for hoisting work, a winch configured to rotate a winch drum with a plurality of electric motors and perform winding-up and winding-down of a hoisted load. In the winch for the hoisting work, it is assumed that a failure occurs in any one of the plurality of electric motors. In this case, a hoisting ability of the crane is deteriorated compared with a hoisting ability at the time when all the electric motors are normally operating without failing.
However, in the conventional overload safety device, the deterioration in the hoisting ability due to the failure of the electric motor of the winch for hoisting work is not assumed. Therefore, when the overload safety device is used in the crane mounted with the winch for hoisting work, it is likely that the hoisting work is performed in a state in which application of a load (a hoisting load) exceeding the hoisting ability deteriorated by the failure of the electric motor is allowed. That is, certainty of overload prevention is spoiled.
On the other hand, when the certainty of the overload prevention is considered important, it is also conceivable to cause the overload safety device to immediately stop the operation of all the electric motors by reducing a set hoisting ability value (a limit value), which is a determination reference for overload prevention, to 0 at a point in time when any one of the plurality of electric motors fails. However, in this case, even if the remaining electric motors other than the failed electric motor are normally operable, the operation of the remaining electric motors is also stopped. That is, even if a certain degree of hoisting ability can be exhibited by rotating the winch drum with the normally operable remaining electric motors, the hoisting work cannot be carried out at all.